Fix the way how btr_free_externally_stored_field() is called in purge.

btr_free_externally_stored_field(): Add parameter field_ref that points
directly to the BLOB reference.  Use rec, offsets, page_zip, and i
only for the page_zip_write_blob_ptr() call.

row_purge_upd_exist_or_extern(): Do not assume that the undo log contains
the entire record.  Only pass the BLOB reference to
btr_free_externally_stored_field().